The semiconductor industry continues to develop lithographic technologies, which can print ever-smaller integrated circuit dimensions. These systems must have high reliability, cost effective throughput, and reasonable process latitude. The integrated circuit fabrication industry has recently changed over from mercury G-line (436 ram) and I-line (365 nm) exposure sources to 248 nm and 193 nm excimer laser sources. This transition was precipitated by the need for higher lithographic resolution with minimum loss in depth-of-focus.
The demands of the integrated circuit industry will soon exceed the resolution capabilities of 193 nm exposure sources, thus creating a need for a reliable exposure source at a wavelength significantly shorter than 193 nm. An excimer line exists at 157 nm, but optical materials with sufficient transmission at this wavelength and sufficiently high optical quality are difficult to obtain. Therefore, all-reflective imaging systems may be required. An all reflective optical system requires a smaller numerical aperture (NA) than the transmissive systems. The loss in resolution caused by the smaller NA can only be made up by reducing the wavelength by a large factor. Thus, a light source in the range of 10 to 20 nm is required if the resolution of optical lithography is to be improved beyond that achieved with 193 nm or 157 nm. Optical components for light at wavelengths below 157 nm are very limited. However, effective incidents reflectors are available and good reflectors multi-layer at near normal angles of incidence can be made for light in the wavelength range of between about 10 and 14 nm. (Light in this wavelength range is within a spectral range known as extreme ultraviolet light and some would light in this range, soft x-rays.) For these reasons there is a need for a good reliable light source at wavelengths in this range such as of about 13.5 nm.
The present state of the art in high energy ultraviolet and x-ray sources utilizes plasmas produced by bombarding various target materials with laser beams, electrons or other particles. Solid targets have been used, but the debris created by ablation of the solid target has detrimental effects on various components of a system intended for production line operation. A proposed solution to the debris problem is to use a frozen liquid or liquidfied or frozen gas target so that the debris will not plate out onto the optical equipment. However, none of these systems have so far proven to be practical for production line operation.
It has been well known for many years that x-rays and high energy ultraviolet radiation could be produced in a plasma pinch operation. In a plasma pinch an electric current is passed through a plasma in one of several possible configuration such that the magnetic field created by the flowing electric current accelerates the electrons and ions in the plasma into a tiny volume with sufficient energy to cause substantial stripping of outer electrons from the ions and a consequent production of x-rays and high energy ultraviolet radiation. Various prior art techniques for generation of high energy radiation from focusing or pinching plasmas are described in the background section of U.S. Pat. No. 6,452,199.
Typical prior art plasma focus devices can generate large amounts of radiation suitable for proximity x-ray lithography, but are limited in repetition rate due to large per pulse electrical energy requirements, and short lived internal components. The stored electrical energy requirements for these systems range from 1 kJ to 100 kJ. The repetition rates typically did not exceed a few pulses per second.
What is needed are production line reliable, systems for producing collecting and directing high energy ultraviolet x-radiation within desired wavelength ranges which can operate reliably at high repetition rates and avoid prior art problems associated with debris formation.